This invention relates to rotary anodes for X-ray tubes comprised of a basic body and made of a molybdenum alloy.
Of the total electric energy supplied to a rotary anode for producing X-radiation, only about 1% is converted into X-rays: the remaining 99% is converted into heat. Therefore, materials for rotary anodes have to be suitable for producing the required X-radiation and, at the same time, exhibit high thermal stability.
Tungsten has both a very high melting point and high ordinal number in the periodic system and thus was found to be highly suitable as a material for the focus path producing the X-rays or the basic body of rotary anodes. However, the drawback of tungsten is that it is very difficult to shape; furthermore, its specific weight is very high, so that the rate of acceleration and deceleration of such rotary anodes can only be relatively low. The centrifugal forces occurring during rotation are, however, very high, so that the maximum permissible number of revolutions of said rotary anodes is relatively low.
In the past, in order to avoid said drawbacks, manufacturers have changed the make-up of such rotary anodes by using tungsten or a tungsten/rhenium alloy only for producing the track of the focal spot, which emits the X-rays, whereas molybdenum or molybdenum alloys, which are substantially lighter in weight than tungsten, are used for manufacturing the basic body of the rotary anodes.
For example, the use of an alloy based on molybdenum with additions of from 0.5 to 50% tungsten, tantalum, niobium, rhenium and/or osmium for producing the basic body of rotary anodes is described in Austrian Patent AT-PS No. 248,555. However, when said rotary anodes are highly stressed, the difference in thermal expansion between the basic body made of the molybdenum alloy and the track of the focal spot made of tungsten or tungsten alloy has an adverse effect, which may cause cracking of the track of the focal spot, these cracks may extend into the basic body.
Austrian patent AT-PS 257,751 describes a rotary anode for X-ray tubes with a basic body consisting of a molybdenum alloy containing 0.05 to 1.5% by weight titanium and additionally, if need be, up to 0.5% by weight zirconium or 0.3% by weight carbon. Said molybdenum alloy is known as "TZM". By using "TZM" as the material for the basic body it was possible to significantly reduce the tendency of cracking.
However, the track of the focal spot for these types of rotary anodes may become distorted in the course of time. Such distortion is more frequent and serious at higher operating temperatures and for larger diameter rotary anodes. Distortion (which may be so minor that it cannot be noticed by the naked eye) results in a reduced yield of X-radiation because a portion of the X-radiation is cut off along the periphery of the X-ray window.
Accordingly, it is an object of the present invention to provide a rotary anode for X-ray tubes in which the basic body comprises a molybdenum alloy but wherein the known problems of said alloy, such as cracking or distortion of the rotary anode, are reduced or avoided.